1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for an internal combustion engine, wherein the control apparatus can implement a locking function for locking the rotation phase of a camshaft of the engine is relative to the crankshaft of the engine, with the rotation phase being locked at approximately the center of its adjustment range.
In the following, the (adjustable) rotation phase of a camshaft relative to the crankshaft of an engine is referred to as the variable cam timing phase, abbreviated to “VCT phase”.
2. Description of Related Art
As described for example in Japanese patent first publication No. 9-324613, and Japanese patent first publication No. 2001-159330, it has been proposed to utilize a hydraulic (i.e., operated by oil pressure) type of variable valve timing apparatus in which, when the engine is halted the VCT phase (for example of the camshaft which operates the air intake valves) becomes locked at a phase angle referred to as the intermediate lock phase, which is at approximately the center of the range of adjustment of the VCT phase. This is done to ensure that when the engine is started, the valve timing will be appropriate for the starting operation. Following engine starting, when the oil pressure rises to a sufficient level, the VCT phase is released from the locked condition and thereafter is controlled in accordance with a target value that is determined in accordance with the operating conditions of the engine. In the unlocked condition, the VCT phase may be adjusted by varying respective oil pressures within chambers (an advancement chamber and a retardation chamber) that are located on circumferentially opposite sides of a member (vane) which rotates with the camshaft.
Locking is effected by setting a lock pin in a specific position, e.g., by a spring which urges the lock pin (after the VCT phase has been brought close to the intermediate lock phase) such as to prevent relative rotation between the crankshaft and the camshaft. Lock release is effected by applying oil pressure such as to overcome the force of the spring, so that the lock pin is moved to a position whereby relative rotation between the crankshaft and the camshaft is enabled, so that the VCT phase can be adjusted.
For example, the lock pin may be mounted for sliding motion in a member fixedly attached to the timing sprocket or timing pulley (which is driven from the crankshaft), with a corresponding hole (referred to in the following as the lock hole) being formed in a member fixedly attached to the camshaft, and with respective radial positions of the lock pin and the lock hole being such that the lock pin can be driven to protrude into the lock hole when the VCT phase is close to the intermediate lock phase. Relative rotation between the camshaft and the timing sprocket (or timing pulley) is thereby prevented, so that the VCT phase is locked at the intermediate lock phase.
In general, such a variable valve timing apparatus is operated utilizing engine oil supplied under pressure from the engine oil pump. When the engine is started with the variable valve timing apparatus in the above-described locked condition, the VCT phase is initially set as the intermediate lock phase. Thereafter, as the engine speed increases and the oil pressure increases to a sufficient value (e.g., an “oil lamp activation” engine speed whereby an oil indicator lamp of the vehicle becomes turned on) the locked condition is released. Thereafter, feedback control of the VCT phase is performed, based on a target VCT phase which is determined in accordance with the current running condition of the engine.
During engine idling, when the engine speed is below the oil lamp activation speed so that the oil pressure is low, it is difficult to maintain the VCT phase close to a predetermined value (i.e., which will enable a rapid transition to a higher engine speed, when the idling condition is ended) such as the intermediate lock phase. Hence, it is also usual to utilize the lock pin to lock the VCT phase at the intermediate lock phase during engine idling, in the same manner as when the engine becomes halted.
However with such an apparatus, when the lock pin is actuated to protrude into the lock hole, while oil pressure is being applied for driving the VCT phase to the intermediate lock phase, the tip of the lock pin may become pressed strongly against a side face of the lock hole. This may prevent the lock pin from becoming engaged within the lock hole, or from becoming completely (stably) engaged in the lock hole. Thus the VCT phase may not become securely locked at the intermediate lock phase. When that occurs, a change in the engine running condition may result in the VCT phase becoming inadvertently released from the locked condition, or an excessive load may be imposed on a tip portion of the lock pin, causing deformation of the pin.
Moreover with such an apparatus when oil pressure is applied to drive the lock pin out from the lock hole (for thereby unlocking the VCT phase (o commence feedback control of the VCT phase) the unlocking operation may be unsuccessful. Specifically, the tip of the lock pin may become pressed strongly against a side face of the lock hole, due to the action of feedback control, before the lock pin has been fully withdrawn from the lock hole. The unlocking operation may therefore fail, so that feedback control of the VCT phase cannot be commenced.
To overcome the latter problem, it might be envisaged that the lock release operation could be performed while the target VCT phase is set close to the intermediate lock phase, to thereby prevent the possibility that the tip of the lock pin may become strongly forced against a side face of the lock hole. However, in addition to achieving reliable disengagement of the lock pin, to perform lock release, it is also necessary that the time point at which lock release is achieved can be accurately and reliably detected.
If the lock release operation is performed while the target VCT phase is set close to the intermediate lock phase, then if lock release is successfully achieved, the actual VCT phase will be held close to the intermediate lock phase up until the time point at which lock release occurs, and will remain close to the intermediate lock phase subsequent to that time point. Hence it becomes difficult or impossible to detect the time point at which lock release occurs.
As a result, there will be delays in initiating feedback control for maintaining the VCT phase at a required target value, when the engine speed is to be increased after a period of idling. Thus the responsiveness of the engine, with respect to recovery from the idling condition, will be poor.
Furthermore, control of the VCT phase is not required during the locked condition, so that (i.e., during engine idling) the supplying of oil to fill the advancement angle chamber and retardation angle chamber may be halted during that condition. Alternatively, oil may be supplied to the advancement angle chamber or to the retardation angle chamber at a lowest limit of flow rate, sufficient to maintain the lock pin in a condition of being lightly pressed against a side face of the lock hole, to hold the pin in a steady condition and thereby prevent instability of the VCT phase.
However when the engine temperature is high, the viscosity of the engine oil becomes lowered, so that oil readily leaks through any gaps of the advancement angle chamber and retardation angle chamber. The rate of leakage of the oil between the advancement angle chamber and retardation angle chamber may exceed the rate at which oil is being supplied. In that case, if the locked condition is continued for a long period of time, the amounts of oil within the advancement angle chamber and retardation angle chamber may become excessively low, with the oil pressure in these chambers thereby becoming excessively low. As a result, when a lock release command is issued and the locked condition is then released, the oil pressure within the chambers may be insufficient to maintain the VCT phase, so that the VCT phase may suddenly change in a direction which is opposite to that required for moving to the target VCT phase.
This presents problems, for example by preventing a smooth transition from the idling condition to acceleration of the engine.
To overcome this, it would be possible to continue to supply oil to the advancement angle chamber and retardation angle chamber so long as the locked condition continues. However this may impose an excessive load on the lock pin, lowering its durability. Moreover when the engine is idling, the pressure at which oil is supplied from the oil pump becomes inherently lower. Thus if oil continues to be supplied to the advancement angle chamber and retardation angle chamber in the locked condition, during engine idling, the is pressure of the supply from the oil pump may become excessively low. Specifically, the pressure may become insufficient for driving other hydraulic equipment of the vehicle, so that the operation of such other hydraulic equipment may be adversely affected.
It is an objective of the present invention to overcome the problems described above.